Are those other devices set to leave the messages on the server after they are accessed? Try sending yourself a message then accessing them on the other computer. Then access the account on the web mail page to see if they are on the server to be downloaded by Thunderbird.

Everything leaves messages on the server. If necessary, I'll go in and manually delete messages from the server if they become a space issue. So far, this has not been a problem.

I'm thinking Google is doing something that causes Thunderbird an issue. What is the technique TB uses to determine if a message is "New" or not. Several months ago, it was necessary to download messages in batches of 250-275 messages. Prior to this limitation TB would download all messages new to the particular computer downloading.

Performed the following to confirm the issue --Create a message on smartphone to myself.Read message on smart phone.Use browser to view message on google server. Message is not bold indicating it has been read.Start Thunderbird. Get Messages.Message is NOT downloaded.Exit ThunderbirdGo back to message using browser.Mark message as NOT ReadMessage is BOLD in inbox on serverStart Thunderbird. Get messages. "No new messages"The message is still on the google server

The question - what characteristics are associated with the message as it resides on the server that Thunderbird can detect if the message has been read previously. This seems to be what is preventing it from being downloaded.

If I don't get an answer here, I will enter this as a Thunderbird bug.

If I don't get an answer here, I will enter this as a Thunderbird bug.

Just to make sure things are clear, we are not Mozilla. This forum is not run by or formally associated with Mozilla. We're an independent user-helping-user community.

I can not recreate your results. I shut down Thunderbird and then composed a new message in Gmail (using a browser) and sent it to my ISP email account. Before opening Thunderbird I read the message on my phone (ISP's app) and on the web mail page of the ISP (using a browser.) I then launched Thunderbird and found that message as a new, unread message in the Inbox of my ISP account, sent from my Gmail account. It was marked as unread in Thunderbird because, as a POP type account, Thunderbird doesn't know anything about the read/unread status of the message on the server, it's just a new, unread message in the Inbox of my ISP account in TB.

A file in the Thunderbird profile (popstate.dat) keeps track of which messages have been downloaded previously and should download a "new" message whether or not it has been read on the server or other device. You could use a browser and try moving all messages in the server inbox folder to another server folder. Then, with Thunderbird closed, rename the file popstate.dat to popstatedat.old. Launch Thunderbird and see if the issue appears with the first new messages received after this procedure is completed.

I'm glad you found a solution but normally enabling both POP and IMAP causes no problem. I've had that configuration for many years. For a while I even had both a POP and a IMAP account in Thunderbird for the same Gmail email address, with no problems.

Update on one account. It appears that juggling settings, all mail was downloaded. However, I discovered that mail that had been read on the smartphone was not being downloaded. The solution used gmail "recent" mode. If your username was "example.email@gmail.com", make it "recent:example.email@gmail.com", for example.The messages for the last 30 days will be downloaded. There may be the possibility that you end up with duplicate messages. There is and add-on "Remove Duplicate Messages (Alternate)" that is GREAT!

I think everything is now working as expected. With "Recent" mode, it is necessary to remember the limitation of only the last 30 days of messages will be downloaded.

I have not done an analysis of the various setting that work and do not. At the moment, everything is working.